1. Field of the Invention
The present invention provides a method for producing homo- and co-polymers of ethylene, or more particularly a method for producing homo- and co-polymers of ethylene using a high activity catalyst to produce said polymers with high bulk densities and narrow molecular weight distributions.
2. Description of the Related Art
Catalysts containing magnesium for polymerization or co-polymerization of ethylene are known to have very high catalytic activities and to produce polymers with high bulk densities. These catalysts are suitable for liquid phase or gas phase polymerization. Liquid phase polymerization of ethylene denotes a polymerization process performed in a medium such as bulk ethylene, isopentane, or hexane. One of the important characteristics of catalysts used in this process is high catalytic activity. The properties of the catalysts will also affect bulk density of the resultant polymers, etc. The molecular weight distribution of the polymer is also an important variable since it helps to determine the physical properties of the polymer. In particular, a narrow molecular weight distribution for ethylene polymers is a very important and advantageous characteristic with respect to injection processed goods.
Many titanium-based catalysts containing magnesium for olefin polymerization, and the manufacturing methods thereof have been reported. Many processes using magnesium solutions to produce catalysts that can generate olefin polymers with a high apparent bulk density are known. A magnesium solution may be obtained by reacting magnesium compounds with electron donors as alcohols, amines, cyclic ethers, or organic carboxylic acids in the presence of a hydrocarbon solvent. Examples using an alcohol are disclosed in U.S. Pat. Nos. 4,330,649 and 5,106,807. Furthermore, methods for production of catalysts containing magnesium by reacting a liquid-phase magnesium solution with a halogenated compound such as titanium tetrachloride are well known. Moreover, there have been attempts to control polymerization activity or molecular weight distribution by adding ester compounds. Such catalysts produce polymers with high bulk densities, but there are still improvements to be made with respect to catalytic activity and molecular weight distribution of the polymer product. Moreover, tetrahydrofuran, a cyclic ester, has been used as a solvent for a magnesium compound in U.S. Pat. Nos. 4,477,639 and 4,518,706.
U.S. Pat. Nos. 4,847,227, 4,816,433, 4,829,037, 4,970,186, and 5,130,284 teach the use of electron donors such as dialkylphthalate, phthaloyl chloride, etc. for reaction with a titanium chloride compound in the production of olefin polymerization catalysts exhibiting superior polymerization activity, which are also capable of enhancing the bulk density of the resultant polymers.
U.S. Pat. No. 5,459,116 teaches a method of production of a titanium solid catalyst by contact-reacting a magnesium solution containing an ester having at least one hydroxyl group as an electron donor with a titanium compound. By this method, a high activity polymerization catalyst was obtained, which produces polymers with high bulk densities, but there is still room for more improvement.
During polymerization of α-olefins, particularly, during polymerization of propylene, the external electron donors are generally used for increasing catalytic activity and stereo-regularity. External electron donors include organic compounds containing oxygen, silicon, nitrogen, sulfur, and phosphorus atoms, such as organic acids, organic anhydrides, organic acid esters, alcohols, ethers, aldehydes, ketones, silanes, amines, aminoxides, amides, diols, and phosphate esters.